Submontane sections of riparian forest habitats are potential focal points of biodiversity but are often the least conserved units of riparian habitats. This is especially concerning in the Northeast Forest Region of China, which has undergone drastic deforestation and river alteration. Effective evaluation and supervision of the suitability of riparian forest habitat and species richness are labor- and time-consuming and could be compensated by using indicator species. In this work, we verified whether the endangered Scaly-sided merganser (SSME) can be used as a qualified indicator of the habitat characteristics and biodiversity of submontane river valleys in the Changbai Mountains area of China. Environmental variables and the richness of riparian species were compared between the river sections settled and unsettled by the SSME. Breeding SSME occurred in river sections with wider channels, larger gravel bars, greater coverage of riverine forests and showed a more concentric inhabitation pattern in wider, deeper, more sinuous and forested riparian habitats. In addition, the probability of occurrence and abundance of the SSME was positively correlated with the species richness of birds, fish and macroinvertebrates as well as the total mass of sampled fish. The correlation of the SSME with the richness of riparian animals and featured riparian habitats and its high public attention profile make it a good candidate for an umbrella and flagship species, which could be an especially valuable surrogate for the protection and monitoring of the habitats and species in the Changbai Mountains area and, probably, other habitats in Northeast Asia. This study adds to the evidence supporting the effectiveness of piscivorous species in indicator roles, suggesting that the indicator roles of mergansers and other fish-eating species deserve verification to complementarily assess important habitats that are difficult to monitor by other means in riparian ecosystems. © 2021 The Author(s)